Viscosity controlled wellbore fluids comprising gels, such as water soluble polymer gels and viscoelastic surfactant gels, are used for various purposes in wellbore operations, such as drilling, completion, production and sequestration or other operations, particularly during hydrocarbon recovery operations. These viscosity controlled fluids must be adapted to form high viscosity fluids for operations such as fracturing, but must also be adapted for “breaking” or reducing their viscosity for subsequent operations, such as hydrocarbon recovery operations.
Fracturing fluids are complex and must simultaneously provide high temperature stability at high pump rates and fluid shear rates that may tend to cause the fluids to degrade and prematurely settle out constituents, such as proppant, before the fracturing operation is complete. Various fracturing fluids have been developed, but most commercially used fracturing fluids are aqueous-based liquids or fluids that have either been gelled or foamed using a gelling agent. Polymeric gelling agents, such as solvatable polysaccharides that are gelled by crosslinking to increase viscosity have been used. Non-polymeric viscoelastic surfactant (VES) gelling agents have also been used. In many cases, VES materials are advantageous compared to polymer gelling agents because they employ low molecular weight surfactants rather than high molecular weight polymers and may leave less gel residue within the pores of oil producing formations, leave no filter cake on the formation face and minimal amounts of residual surfactant coating the proppant, and inherently do not create microgels or fish-eye-type polymeric masses. However, both water soluble polymer and VES systems are widely used.
Both water soluble polymer and VES materials require breaker systems for to reduce their viscosity after use. These have generally included both internal and external breaker systems, which utilize reservoir conditions for viscosity reduction (breaking) and fluid removal (clean-up) during hydrocarbon production. While useful, these breaker systems have limitations, including incomplete removal of the gelled fluids, resulting in residual formation damage (e.g., impairment of hydrocarbon production). Post-treatment clean-up fluids have been employed in an attempt to break the gelled fluid for removal, but their effectiveness has been limited, resulting in well sections with unbroken or poorly broken gelled fluid that impairs hydrocarbon production, or in production delays associated with instances where breaking and clean-up takes a long time, such as several days up to possibly months to break and then produce the VES treatment fluid from the reservoir.
Various internal breakers that are activated within the fluid, such as by downhole temperatures, have also been used with the water soluble polymer-gelled and VES-gelled fluids, and typically allow a controlled rate of gel viscosity reduction in 1 to 8 hours.
While these internal breakers are very useful, the development of additional internal breakers to provide enhanced control of the breaking of water soluble polymer-gelled and VES-gelled fluids is very desirable, particularly in view of the widespread use of these fluids in fracturing and other downhole operations.